1. Technical Field
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing an image on a recording medium and an image forming apparatus incorporating the fixing device.
2. Description of the Background
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a development device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device may employ an endless belt or an endless film to heat the recording medium. For example, as disclosed by JP-2004-286922-A, the fixing device includes the endless belt looped over a heating roller and a nip formation pad and a pressure roller pressed against the nip formation pad via the belt to form a fixing nip between the pressure roller and the belt. As the recording medium bearing the toner image is conveyed through the fixing nip, the belt and the pressure roller apply heat and pressure to the recording medium, melting and fixing the toner image on the recording medium.
The belt is requested to be heated quickly to shorten a first print time taken to output the recording medium bearing the fixed toner image upon receipt of a print job. Additionally, as the image forming apparatus conveys an increased amount of recording media at high speed, the belt is requested to overcome shortage of heat.
To address those requests, the fixing device may include the endless film. For example, as disclosed by JP-H4-044083-P, a pressure roller is pressed against a heater disposed inside a loop formed by the film via the film to form a fixing nip between the pressure roller and the film. As the recording medium bearing the toner image is conveyed through the fixing nip, the film and the pressure roller apply heat and pressure to the recording medium, melting and fixing the toner image on the recording medium. Since the film is heated by the heater situated at the fixing nip, the film is heated insufficiently at an entry to the fixing nip, resulting in faulty fixing. Accordingly, the film is requested to overcome shortage of heat at the entry to the fixing nip.
To address those requests, the fixing device may employ a metal thermal conductor as disclosed by JP-2007-334205-P. FIG. 1 illustrates a fixing device 20R disclosed by JP-2007-334205-P. A tubular, metal thermal conductor 302R is disposed inside an endless belt 301R. The endless belt 301R is slidable over the metal thermal conductor 302R. A heater 303R is located inside the metal thermal conductor 302R. The heater 303R heats the metal thermal conductor 302R which in turn heats the endless belt 301R. A pressure roller 304R is pressed against the metal thermal conductor 302R via the endless belt 301R to form a fixing nip N between the pressure roller 304 and the endless belt 301R. As the pressure roller 304R rotates clockwise in FIG. 1, the endless belt 301R rotates counterclockwise in FIG. 1 in accordance with rotation of the pressure roller 304R, thus conveying a recording medium P bearing a toner image in a recording medium conveyance direction D1. Since the tubular, metal thermal conductor 302R is disposed opposite the endless belt 301R throughout the entire circumferential span of the endless belt 301R, the metal thermal conductor 302R heats the endless belt 301R quickly, thus shortening the first print time and overcoming shortage of heat.
In order to shorten the first print time further, the fixing device may employ an endless belt directly heated by a heater disposed inside or outside the endless belt as disclosed by JP-2008-058833-A and JP2008-139779-A. However, since a recording medium is not conveyed over the endless belt at both axial ends of the endless belt and therefore does not draw heat from both axial ends of the endless belt, both axial ends of the endless belt are susceptible to overheating. To address this circumstance, a heat shield may be interposed between the heater and the endless belt to shield the endless belt from the heater.
However, as the heat shield is heated by the heater, the heat shield and a resin flange situated in proximity to the heat shield are susceptible to overheating that may result in thermal deformation of the heat shield and the flange.